It should take less wood to make trapdoors or you should get more trapdoors per crafting recipe.

So, instead of getting 2 trapdoors from a crafting recipe that uses 6 wood blocks, you should get 4 or 6 trapdoors, just like you get 3 doors for 6 blocks.

Logic: you can fill the same space as a door with two trapdoors, so they should take half the amount of wood to craft, not twice the number of blocks. If it takes 2 blocks per door, it should take one block per trapdoor.